It is generally known that small children will occasionally make unauthorized, unsupervised and undesirable entry into a common household toilet presumably for the purpose of gaining access to the standing water therein. Several devices have been proposed to prevent such unauthorized access to the toilet. However, none of these prior art devices have achieved any significant acceptance in the marketplace, either because they are ineffective in preventing entry into the toilet; they are unduly expensive to manufacture; they are unduly complicated in their operation so as to make even ordinary use of the toilet by adults inconvenient; or they are not suitable for retrofitting to existing toilet facilities.